I have a developer workstation that has registered DDR2 ECC DIMMs already installed. I'm planning on getting some more RAM. Can I add memory modules that aren't ECC and/or registered? I mean, does the machine work with both ECC and non-ECC modules installed? I guess registered ECC memory is at least more expensive?


ECC and non-ECC can be mixed on the same motherboard if the motherboard does not require ECC. (Funny, I just tried it this morning.) Your BIOS also has to be able to force the ECC to operate in non-ECC.

Do you know if the motherboard requires ECC? What's the model?

  • 2
    Some chipsets do not like mixing ECC and non ECC RAM, some will but will run it all as non-ECC, and I believe a few will use both and use the ECC to its full potential. Check your manual before trying. Aug 11 '09 at 15:30
  • Definitely check if the MB supports it. I tried it on my work machine a little while ago and it wouldn't boot (also caused the BIOS settings to revert to factory settings).
    – Herms
    Aug 11 '09 at 18:31
  • @hyperslug: I found the motherboard manual and it says: "The D1691 supports DDR-II Registered type memory modules and requires minimum two memory modules to power up the system. Unbuffered or DDR-I Memory is NOT supported." So, I guess unbuffered means non-registered?
    – Kaitsu
    Aug 12 '09 at 7:05
  • 1
    Yes, unbuffered = non-registered. From the Technical manual "D1691 supports up to eight 240-pin 1.8-Volt DDR-II 400 Registered ECC DIMM modules", it sounds like anything non-ECC or non-Registered won't work. My particular motherboard wouldn't even boot.
    – hyperslug
    Aug 12 '09 at 21:27
  • When making a mistake buying memory for Xeon and Opteron Proliant servers, I found out that registered is not the same as ECC. en.wikipedia.org/wiki/Registered_memory
    – paradroid
    Dec 2 '10 at 2:47

I had this problem with an HP server in the past. You can make it work, but it is a hit or miss thing. It is very dependant on the memory modules being mixed. Some work together, some do not. I mixed and matched a few different combinations. Some did work, others did not. In the end I decided to cut my losses and just move forward because the larger modules I had did not play well with the system. In my case I bought non ECC memory to replace the small sticks of ECC the system came with. They gave me a whole 256 MB of ram with that server so keeping the ECC was hardly worth doing. The new memory worked fine and the only thing that it did was complain that non HP memory was being used.

My advice is just buy a uniform memory module type once you find one that works. Ditch the ECC and just replace all of it. You could have anomalies due to the variances between the sticks and their performance capabilities.


I have heard it will work if the motherboard is ok with it, but it is not advised or recommended by anyone. Using mixed RAM also defeats the purpose of using ECC Ram in the first place though.

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